The Collaborative International Dictionary of English v.0.48:

Decadent \De*ca"dent\, a.
   Decaying; deteriorating.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decadent \De*ca"dent\, n.
   One that is decadent, or deteriorating; esp., one
   characterized by, or exhibiting, the qualities of those who
   are degenerating to a lower type; -- specif. applied to a
   certain school of modern French writers.

         The decadents and [ae]sthetes, and certain types of
         realists.                                --C. L. Dana.

         The business men of a great State allow their State to
         be represented in Congress by "decadents". --The
                                                  Century.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

decadent
    adj 1: marked by excessive self-indulgence and moral decay; "a
           decadent life of excessive money and no sense of
           responsibility"; "a group of effete self-professed
           intellectuals" [syn: decadent, effete]
    n 1: a person who has fallen into a decadent state (morally or
         artistically)

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "decadent":
   abandoned, coming apart, contaminated, corrupt, corrupted,
   cracking, crumbling, debased, debauched, decaying, declining,
   degenerate, degenerating, degenerative, degraded, depraved,
   deteriorating, disintegrating, dissipated, dissolute, draining,
   drooping, dwindling, ebbing, effete, fading, failing, falling,
   falling off, flagging, fragmenting, going to pieces, immoral,
   languishing, marcescent, morally polluted, on the wane, perverted,
   pining, polluted, profligate, regressive, reprobate, retrograde,
   retrogressive, rotten, self-indulgent, shriveling, sinking,
   sliding, slipping, slumping, steeped in iniquity, subsiding,
   tabetic, tainted, vice-corrupted, vitiated, waning, warped,
   wasting, wilting, withering, worsening